Olive. Mitotic division of the luiclei of tlie Cvanopliyceae. 27 



otliers say tliat its absence. along witli that of the nuclear mem- 

 brane is only proof of tlie primitive nature of the cyanophyceous 

 nneleus. AVager, liowever, says that, under eertain conditions, 

 tlie chromatin siibstance of the central body is found Condensed 

 into a Single deeply stained g-ranule suspended by delicate fibers 

 in the center of the cell. I have not seen such a structure as 

 he describes in the forms which I have examined, but, in the 

 resting nuclei of spores and heterocysts, some of the chromatin 

 graniües are sometimes larger than others and might readily 

 pass for nucleoli. Many nucleoli among the lower plants and 

 the Protozoa are undoubtedly merely large masses or granules 

 of chromatin; and, if we coiüd prodnce by experiment resting 

 nnclei in the vegetative cells of the CyanopTnjceae, it is possible 

 that we wotild have also in these plants such a concentration 

 of chromatin substance. 



The resting: nuclei in spores and Iieterocysts. 



The nuclei of spores and heterocysts are of special interest 

 in that they furnish another point of evidence in support of the 

 conclusion that there is no wide and nnsurmonntable difference 

 between the nnclei of the Cyanophyceae and those of higher 

 plants. It will be seen that the nticlei in figs. 49 and 91, 

 representing young heterocysts of CaJothrlx and Cylindrospermum 

 respectively ; as well as the nnclei sliown in hg. 58, a spore of 

 Nostoc] and in 93, a spore of Cylindrospermum; and especially 

 those in hgs. 100 — 103, cross sections of young spores of Cylindro- 

 spermum, resemble closely the resting nnclens with which we 

 are familiär. A cavity, in which we see chromatin granules, 

 and a more or less clearly defined, delicate, nuclear membrane, 

 now contribute to the resemblance, which was lacking in the 

 vegetative stages. 



Both Hegler and Kohl come rightly to the conclnsion 

 that the cell contents of heterocysts become hnally disorganized, 

 and that the nucleus, chromatophore, cyanophycin granules, and 

 slime globules gradually disappear. In fig. 77, which is stained 

 with methylene blne, each of the vegetative cells of the filament 

 of Cyluidrospprimim shows one or more slime globnles, the spore 

 one only, while the yoimg heterocyst has two exceedingly 

 minnte, reddish slime globules in a bluish background. AVhen 

 older, we find in heterocysts no indication whatever of any 

 granulär contents, except the disorganized chronuitin of the dead 

 nucleus (note the heterocysts of figs. 37, 38, 50, 53, 80, 88). It 

 is highly important to note, however, that in the young hetero- 

 cysts of Calotlirix and Cylindrospormum , before disorganization 

 occurs, the nucleus apparently begins to enter a normal resting 

 Kondition and to form a nuclc^xr vacu()l(\ 



The mature spore of Cyliiidrospenniun shows one reniarkably 

 cnrious feature, which to the writer remained for a long time 

 an inexplicable puzzle. It will be noted that in fig. 77, a half 

 matured spore of Cylindrospcr)ini m . tlic nndtitude of cyanophycin 



